Thompson is expected to work out of the bullpen for now, John Denton of MLB.com reports.
Thompson has made one start and two relief appearances thus far in his career. He was knocked around for five runs in five innings by the Pirates in his lone start but allowed just one run in a combined seven innings as a reliever. His next relief outings may be shorter, however, as the Cardinals appear set to match him up against other teams' toughest lefties rather than use him in a multi-inning role.